The green colour of leaves is almost wholly dependent on it; and
although some flowers will become fully coloured in the dark, others
are decidedly affected by the absence of light, even when the
foliage is fully exposed to it. Looking therefore at the numerous
colouring matters which are developed in the tissues of plants, the
sensitiveness of these pigments to light, the changes they undergo
during growth and development, and the facility with which new
chemical combinations are effected by the physiological processes of
plants as shown by the endless variety in the chemical constitution
of vegetable products, we have no difficulty in comprehending the
general causes which aid in producing the colours of the vegetable
world, or the extreme variability of those colours. We may therefore
here confine ourselves to an inquiry into the various uses of colour
in the economy of plants; and this will generally enable us to
understand how it has become fixed and specialised in the several
genera and species of the vegetable kingdom.
_Protective Coloration and Mimicry in Plants._--In animals, as we
have seen, colour is greatly influenced by the need of protection
from, or of warning to, their numerous enemies, and by the necessity
for identification and easy recognition. Plants rarely need to be
concealed, and obtain protection either by their spines, their
hardness, their hairy covering, or their poisonous secretions.
A very few cases of what seem to be true protective colouring
do, however, exist; the most remarkable being that of the “stone
mesembryanthemum,” of the Cape of Good Hope, which, in form and
colour closely resembles the stones among which it grows; and
Dr. Burchell, who first discovered it, believes that the juicy
little plant thus generally escapes the notice of cattle and wild
herbivorous animals. Mr. J. P. Mansel Weale also noticed that
many plants growing in the stony Karoo have their tuberous roots
above the soil; and these so perfectly resemble the stones among
which they grow that, when not in leaf, it is almost impossible
to distinguish them (_Nature_, vol. iii. p. 507). A few cases of
what seems to be protective mimicry have also been noted; the most
curious being that of three very rare British fungi, found by Mr.
Worthington Smith, each in company with common species which they so
closely resembled that only a minute examination could detect the
difference. One of the common species is stated in botanical works
to be “bitter and nauseous,” so that it is not improbable that the
rare kind may escape being eaten by being mistaken for an uneatable
species, though itself palatable. Mr. Mansel Weale also mentions a
labiate plant, the _Ajuga-ophrydis_, of South Africa, as strikingly
resembling an orchid. This may be a means of attracting insects to
fertilize the flower in the absence of sufficient nectar or other
attraction in the flower itself; and the supposition is rendered
more probable by this being the only species of the genus Ajuga in
South Africa. Many other cases of resemblances between very distinct
plants have been noticed--as that of some Euphorbias to Cacti; but
these very rarely inhabit the same country or locality, and it has
not been proved that there is in any of these cases the amount of
inter-relation between the species which is the essential feature
of the protective “mimicry” that occurs in the animal world.
The different colours exhibited by the foliage of plants and the
changes it undergoes during growth and decay, appear to be due to
the general laws already sketched out, and to have little if any
relation to the special requirements of each species. But flowers
and fruits exhibit definite and well-pronounced tints, often varying
from species to species, and more or less clearly related to the
habits and functions of the plant. With the few exceptions already
pointed out, these may be generally classed as _attractive_ colours.
_Attractive Colours of Fruits._--The seeds of plants require to be
dispersed, so as to reach places favourable for germination and
growth. Some are very minute, and are carried abroad by the wind;
or they are violently expelled and scattered by the bursting of the
containing capsules. Others are downy or winged, and are carried
long distances by the gentlest breeze; or they are hooked and
stick to the fur of animals. But there is a large class of seeds
which cannot be dispersed in either of these ways, and they are
mostly contained in eatable fruits. These fruits are devoured by
birds or beasts, and the hard seeds pass through their stomachs
undigested, and, owing probably to the gentle heat and moisture to
which they have been subjected, in a condition highly favourable
for germination. The dry fruits or capsules containing the first
two classes of seeds are rarely, if ever, conspicuously coloured;
whereas the eatable fruits almost invariably acquire a bright colour
as they ripen, while at the same time they become soft and often
full of agreeable juices. Our _red_ haws and hips, our _black_
elderberries, our _blue_ sloes, and whortleberries, our _white_
mistletoe and snowberry, and our _orange_ sea-buckthorn, are examples
of the colour-sign of edibility; and in every part of the world the
same phenomenon is found. Many such fruits are poisonous to man
and to some animals, but they are harmless to others; and there is
probably nowhere a brightly-coloured pulpy fruit which does not
serve as food for some species of bird or mammal.
_Protective Colours of Fruits._--The nuts and other hard fruits of
large forest-trees, though often greedily eaten by animals, are
not rendered attractive to them by colour, because they are not
intended to be eaten. This is evident; for the part eaten in these
cases is the seed itself, the destruction of which must certainly
be injurious to the species. Mr. Grant Allen, in his ingenious work
on _Physiological Æsthetics_, well observes that the colours of
all such fruits are protective--green when on the tree, and thus
hardly visible among the foliage, but turning brown as they ripen
and fall on the ground, as filberts, chestnuts, walnuts, beech-nuts,
and many others. It is also to be noted that many of these are
specially though imperfectly protected; some by a prickly coat as
in the chestnuts, or by a nauseous covering as in the walnut; and
the reason why the protection is not carried further is probably
because it is not needed, these trees producing such vast quantities
of fruit, that however many are eaten, more than enough are always
left to produce young plants. In the case of the attractively
coloured fruits, it is curious to observe how the _seeds_ are always
of such a nature as to escape destruction when the fruit itself is
eaten. They are generally very small and comparatively hard, as
in the strawberry, gooseberry, and fig; if a little larger, as in
the grape, they are still harder and less eatable; in the fruit
of the rose (or hip) they are disagreeably hairy; in the orange
tribe excessively bitter. When the seeds are larger, softer, and
more eatable, they are protected by an excessively hard and stony
covering, as in the plum and peach tribe; or they are inclosed in
a tough horny core, as with crabs and apples. These last are much
eaten by swine, and are probably crushed and swallowed without
bruising the core or the seeds, which pass through their bodies
undigested. These fruits may also be swallowed by some of the larger
frugivorous birds; just as nutmegs are swallowed by pigeons for the
sake of the mace which incloses the nut, and which by its brilliant
red colour is an attraction as soon as the fruit has split open,
which it does upon the tree.
There is, however, one curious case of an attractively coloured
seed which has no soft eatable covering. The _Abrus precatoria_,
or “rosary bean,” is a leguminous shrub or small tree growing in
many tropical countries, whose pods curl up and split open on the
tree, displaying the brilliant red seeds within. It is very hard and
glossy, and is said to be, as no doubt it is, “very indigestible.”
It may be that birds, attracted by the bright colour of the seeds,
swallow them, and that they pass through their bodies undigested, and
so get dispersed. If so it would be a case among plants analogous
to mimicry among animals--an appearance of edibility put on to
deceive birds for the plant’s benefit. Perhaps it succeeds only with
young and inexperienced birds, and it would have a better chance
of success, because such deceptive appearances are very rare among
plants.
The smaller plants whose seeds simply drop upon the ground, as in
the grasses, sedges, composites, umbelliferæ, &c., always have dry
and obscurely-coloured capsules and small brown seeds. Others whose
seeds are ejected by the bursting open of their capsules, as with
the oxalis and many of the caryophyllaceæ, scrophulariaceæ, &c.,
have their seeds very small and rarely or never edible.
It is to be remarked that most of the plants whose large-seeded nuts
cannot be eaten without destroying their germinating power--as the
oaks, beeches, and chestnuts--are trees of large size which bear
great quantities of fruit, and that they are long lived and have a
wide geographical range. They belong to what are called dominant
groups, and are thus able to endure having a large proportion
of their seeds destroyed with impunity. It is a suggestive fact
that they are among the most ancient of known dicotyledonous
plants--oaks and beeches going back to the Cretaceous period with
little change of type, so that it is not improbable that they may
be older than any fruit-eating mammal adapted to feed upon their
fruits. The _attractive_ coloured fruits on the other hand, having
so many special adaptations to dispersal by birds and mammals, are
probably of more recent origin.[21] The apple and plum tribes are
not known earlier than the Miocene period; and although the record
of extinct vegetable life is extremely imperfect, and the real
antiquity of these groups is no doubt very much greater, it is not
improbable that the _comparative_ antiquity of the fruit-bearing
and nut-bearing trees may remain unchanged by further discoveries,
as has almost always happened as regards the comparative antiquity
of animal groups.
[21] I owe this remark to Mr. Grant Allen, author of _Physiological
Æsthetics_.
_Attractive Colours of Flowers._--The colours of flowers serve
to render them visible and recognizable by insects, which are
attracted by secretions of nectar or pollen. During their visits
for the purpose of obtaining these products, insects involuntarily
carry the pollen of one flower to the stigma of another, and thus
effect cross-fertilization; which, as Mr. Darwin was the first to
demonstrate, immensely increases the vigour and fertility of the
next generation of plants. This discovery has led to the careful
examination of great numbers of flowers; and the result has been
that the most wonderful and complex arrangements have been found to
exist, all having for their object to secure that flowers shall not
be self-fertilized perpetually, but that pollen shall be carried,
either constantly or occasionally, from the flowers of one plant to
those of another. Mr. Darwin himself first worked out the details in
orchids, primulas, and some other groups; and hardly less curious
phenomena have since been found to occur even among some of the most
regularly-formed flowers. The arrangement, length, and position of
all the parts of the flower is now found to have a purpose, and
not the least remarkable portion of the phenomenon is the great
variety of ways in which the same result is obtained. After the
discoveries with regard to orchids, it was to be expected that the
irregular, tubular, and spurred flowers should present various
curious adaptations for fertilization by insect-agency. But even
among the open, cup-shaped, and quite regular flowers, in which
it seemed inevitable that the pollen must fall on the stigma and
produce constant self-fertilization, it has been found that this is
often prevented by a physiological variation--the anthers constantly
emitting their pollen either a little earlier or a little later
than the stigmas of the same flower, or of other flowers on the
same plant, were in the best state to receive it; and as individual
plants in different stations, soils, and aspects, differ somewhat
in the time of flowering, the pollen of one plant would often be
conveyed by insects to the stigmas of some other plant in a condition
to be fertilized by it. This mode of securing cross-fertilization
seems so simple and easy, that we can hardly help wondering why
it did not always come into action, and so obviate the necessity
for those elaborate, varied, and highly complex contrivances found
perhaps in the majority of coloured flowers. The answer to this
of course is, that _variation_ sometimes occurred most freely in
one part of a plant’s organization, and sometimes in another; and
that the benefit of cross-fertilization was so great that _any_
variation that favoured it was preserved, and then formed the
starting-point of a whole series of further variations, resulting
in those marvellous adaptations for insect-fertilization, which
have given much of their variety, elegance, and beauty, to the
floral world. For details of these adaptations we must refer the
reader to the works of Darwin, Lubbock, Hermann Müller, and others.
We have here only to deal with the part played by colour, and by
those floral structures in which colour is most displayed.
_Attractive Odours in Flowers._--The sweet odours of flowers, like
their colours, seem often to have been developed as an attraction
or guide to insect fertilizers, and the two phenomena are often
complementary to each other. Thus, many inconspicuous flowers--like
the mignonette and the sweet-violet, can be distinguished by their
odours before they attract the eye, and this may often prevent
their being passed unnoticed; while very showy flowers, and
especially those with variegated or spotted petals, are seldom
sweet. White, or very pale flowers, on the other hand, are often
excessively sweet, as exemplified by the jasmine and clematis; and
many of these are only scented at night, as is strikingly the case
with the night-smelling stock, our butterfly orchids (_Habenaria
chlorantha_), the greenish-yellow _Daphne pontica_, and many others.
These white flowers are mostly fertilized by night-flying moths;
and those which reserve their odours for the evening probably
escape the visits of diurnal insects, which would consume their
nectar without effecting fertilization. The absence of odour in
showy flowers, and its preponderance among those that are white,
may be shown to be a fact by an examination of the lists in Mr.
Mongredien’s work on hardy trees and shrubs.[22] He gives a list
of about 160 species with showy flowers, and another list of sixty
species with fragrant flowers: but only twenty of these latter are
included among the showy species, and these are almost all white
flowered. Of the sixty species with fragrant flowers, more than
forty are white, and a number of others have greenish, yellowish,
or dusky and inconspicuous flowers. The relation of white flowers
to nocturnal insects is also well shown by those which, like the
evening primroses, only open their large white blossoms after sunset.
The red Martagon lily has been observed by Mr. Hermann Müller to
be fertilized by the humming-bird hawk moth, which flies in the
morning and afternoon when the colours of this flower, exposed to
the nearly horizontal rays of the sun, glow with brilliancy, and
when it also becomes very sweet-scented.
[22] _Trees and Shrubs for English Plantations_, by Augustus
Mongredien. Murray, 1870.
_Attractive grouping of Flowers._--To the same need of
conspicuousness the combination of so many individually small flowers
into heads and bunches is probably due, producing such broad masses
as those of the elder, the guelder-rose, and most of the Umbelliferæ,
or such elegant bunches as those of the lilac, laburnum, horse
chestnut, and wistaria. In other cases minute flowers are gathered
into dense heads, as with _Globularia_, _Jasione_, clover, and all
the Compositæ; and among the latter the outer flowers are often
developed into a ray, as in the sunflowers, the daisies, and the
asters, forming a star-like compound flower, which is itself often
produced in immense profusion.
_Why Alpine Flowers are so Beautiful._--The beauty of alpine flowers
is almost proverbial. It consists either in the increased size of the
individual flowers as compared with the whole plant, in increased
intensity of colour, or in the massing of small flowers into dense
cushions of bright colour; and it is only in the higher Alps, above
the limit of forests and upwards towards the perpetual snow-line
that these characteristics are fully exhibited. This effort at
conspicuousness under adverse circumstances may be traced to the
comparative scarcity of winged insects in the higher regions, and
to the necessity for attracting them from a distance. Amid the
vast slopes of debris and the huge masses of rock so prevalent in
higher mountain regions, patches of intense colour can alone make
themselves visible and serve to attract the wandering butterfly from
the valleys. Mr. Hermann Müller’s careful observations have shown,
that in the higher Alps bees and most other groups of winged insects
are almost wanting, while butterflies are tolerably abundant; and
he has discovered, that in a number of cases where a lowland flower
is adapted to be fertilized by bees, its alpine ally has had its
structure so modified as to be adapted for fertilization only by
butterflies.[23] But bees are always (in the temperate zone) far
more abundant than butterflies, and this will be another reason why
flowers specially adapted to be fertilized by the latter should be
rendered unusually conspicuous. We find, accordingly, the yellow
primrose of the plains replaced by pink and magenta-coloured alpine
species; the straggling wild pinks of the lowlands by the masses of
large flowers in such mountain species as _Dianthus alpinus_ and
_D. glacialis_; the saxifrages of the high Alps with bunches of
flowers a foot long as in _Saxifraga longifolia_ and _S. cotyledon_,
or forming spreading masses of flowers as in _S. oppositifolia_;
while the soapworts, silenes, and louseworts are equally superior
to the allied species of the plains.
[23] _Nature_, vol. xi. pp. 32, 110.
_Why Allied Species of Flowers Differ in Size and Beauty._--Again,
Dr. Müller has discovered that when there are showy and inconspicuous
species in the same genus of plants, there is often a corresponding
difference of structure, those with large and showy flowers being
quite incapable of self-fertilization, and thus depending for
their very existence on the visits of insects; while the others
are able to fertilize themselves should insects fail to visit
them. We have examples of this difference in _Malva sylvestris_,
_Epilobium augustifolium_, _Polygonum bistorta_, and _Geranium
pratense_--which have all large or showy flowers, and must be
fertilized by insects--as compared with _Malva rotundifolia_,
_Epilobium parviflorum_, _Polygonum aviculare_, and _Geranium
pusillum_, which have small or inconspicuous flowers, and are so
constructed that if insects should not visit them they are able to
fertilize themselves.[24]
[24] _Nature_, vol. ix. p. 164.
_Absence of Colour in Wind-fertilized Flowers._--As supplementing
these curious facts showing the relation of colour in flowers
to the need of the visits of insects to fertilize them, we have
the remarkable, and on any other theory, utterly inexplicable
circumstance, that in all the numerous cases in which plants are
fertilized by the agency of the wind they never have specially
coloured floral envelopes. Such are our pines, oaks, poplars,
willows, beeches, and hazel; our nettles, grasses, sedges, and
many others. In some of these the male flowers are, it is true,
conspicuous, as in the catkins of the willows and the hazel, but
this arises incidentally from the masses of pollen necessary to
secure fertilization, as shown by the entire absence of a corolla
or of those coloured bracts which so often add to the beauty and
conspicuousness of true flowers.
_The Same Theory of Colour Applicable to Animals and Plants._--It
may be thought that this absence of colour where it is not wanted is
opposed to the view maintained in the earlier part of the preceding
chapter, that colour is normal and is constantly tending to appear
in natural objects. It must be remembered, however, that the green
colour of foliage, due to chlorophyll, prevails throughout the
greater part of the vegetable kingdom, and has, almost certainly,
persisted through long geological periods. It has thus acquired a
fixity of character which cannot be readily disturbed; and, as a
matter of fact, we find that colour rarely appears in plants except
in association with a considerable modification of leaf-texture,
such as occurs in the petals and coloured sepals of flowers.
Wind-fertilized plants never have such specially organized floral
envelopes and, in most cases, are entirely without a calyx or
corolla. The connection between modification of leaf-structure and
colour is further seen in the greater amount and variety of colour
in irregular than in regular flowers. The latter, which are least
modified, have generally uniform or but slightly varied colours;
while the former which have undergone great modification, present
an immense range of colour and marking, culminating in the spotted
and variegated flowers of such groups as the Scrophularineæ and
Orchideæ. The same laws as to the conditions of a maximum production
of colour are thus found to obtain both in plants and animals.
_Relation of the Colours of Flowers and their Geographical
Distribution._--The adaptation of flowers to be fertilized by
insects--often to such an extent that the very existence of the
species depends upon it--has had wide-spread influence on the
distribution of plants and the general aspects of vegetation. The
seeds of a particular species may be carried to another country,
may find there a suitable soil and climate, may grow and produce
flowers; but if the insect which alone can fertilize it should not
inhabit that country, the plant cannot maintain itself, however
frequently it may be introduced or however vigorously it may grow.
Thus may probably be explained the poverty in flowering-plants and
the great preponderance of ferns that distinguishes many oceanic
islands, as well as the deficiency of gaily-coloured flowers in
others. This branch of the subject is discussed at some length in my
Address to the Biological Section of the British Association,[25]
but I may here just allude to two of the most striking cases. New
Zealand is, in proportion to its total number of flowering-plants,
exceedingly poor in handsome flowers, and it is correspondingly
poor in insects, especially in bees and butterflies, the two groups
which so greatly aid in fertilization. In both these aspects it
contrasts strongly with Southern Australia and Tasmania in the same
latitudes, where there is a profusion of gaily-coloured flowers
and an exceeding rich insect-fauna. The other case is presented by
the Galapagos islands, which, though situated on the equator off
the west coast of South America, and with a tolerably luxuriant
vegetation in the damp mountain zone, yet produce hardly a single
conspicuously-coloured flower; and this is correlated with, and no
doubt dependent on, an extreme poverty of insect life, not one bee
and only a single butterfly having been found there.
[25] See Chapter VII. of this volume.
Again, there is reason to believe that some portion of the large
size and corresponding showiness of tropical flowers is due to
their being fertilized by very large insects and even by birds.
Tropical sphinx-moths often have their probosces nine or ten inches
long, and we find flowers whose tubes or spurs reach about the same
length; while the giant bees, and the numerous flower-sucking birds,
aid in the fertilization of flowers whose corollas or stamens are
proportionately large.